Fire sensors which detect an outbreak of a fire in their surroundings by sensing heat, smoke, and/or flame generated by the fire have been known. When a fire sensor is installed in an explosion-proof zone which is filled with inflammable gas or explosive gas, an explosion due to ignition of the gas surrounding the fire sensor may be caused by sparks generated inside the fire sensor because of a failure or similar problems of electronic components. Therefore fire sensors to be installed in explosion-proof zones have to be explosion-proof so that they will not cause a far-reaching explosion in their surroundings.
Among fire sensors, smoke detectors in particular, capable of sensing smoke, are especially widely used because they are capable of detecting an outbreak of a fire in its early stage. As a kind of smoke detectors may be mentioned photoelectric smoke detectors. A photoelectric smoke detector has a light emitter and a light receiver that senses light scattered by smoke-containing air, thereby detecting smoke emission.
The photoelectric smoke detector is normally provided with an electronic circuit board to control the operations of the light emitter and the light receiver. The electronic circuit board may sometimes be caused to spark by a temporal flow of overcurrent because of a failure of electronic components, or may have an abnormally high temperature because of deterioration in insulation resistance on the face of the board. The sparks or the abnormal high temperature of the electronic circuit board may ignite inflammable gas, which may result in an explosion.
For example, a photoelectric smoke detector employing a type of protection called an intrinsic safety & intrinsically safe system is disclosed in patent document 1, or Japanese patent No. 3938750. The intrinsic safety & intrinsically safe system-type photoelectric smoke detector suppresses the magnitude of an electric current flowing on the electronic circuit board to prevent the electronic circuit board from generating such an amount of electric sparks on its surface as to ignite a surrounding inflammable gas.
In patent documents 2-4 are disclosed photoelectric smoke detectors where the light emitter and the light receiver are placed at a distance so that an explosion on the surface of the electric circuit board will not be caused.
Patent document 2, for example, discloses “a separate-type photoelectric smoke detector including a smoke detecting section to be placed in a caution zone, the smoke detecting section comprising a light-emitting element and a light-receiving lens which receives light scattered or transmitted by smoke; . . . and an electric circuit section, wherein the electric circuit section is placed at a distance from the smoke detecting section (see the claim for utility model registration of patent document 2).”
Also, patent document 3 teaches from page 3, line 17 to page 4, line 3 thereof: “The device employs the arrangement where a labyrinthine base for introducing smoke is placed at a distance from an electronic circuit including a light-emitting element, a light-receiving element, and electronic components; and the labyrinthine base and the electronic circuit are connected with optical fibers. This arrangement makes it possible to form a photoelectric smoke detector by placing only the labyrinthine base, which is not affected by high temperatures, in a high temperature room, thereby providing a smoke detector capable of detecting smoke even in a high temperature atmosphere without hindrance.”
Furthermore, patent document 4 discloses from the last line in the upper right column to the fifth line in the lower left column on page 2: “The smoke detector employs an arrangement where a light emitter and light receivers for receiving direct light and scattered light are housed in a metal case of a circuit section. The emitter and the receivers are connected with a black box of the smoke sensor by optical fibers respectively. The black-box-side end of each of the optical fibers is provided with a lens for emitting or receiving light, thereby detecting smoke.”
The photoelectric smoke detector using optical fibers, as disclosed in patent documents 2-4, has an arrangement where an electronic circuit board housed in an explosion-proof enclosure and the light emitting and receiving sections for detecting smoke which are wired to the electronic circuit board by means of cable glands are completely separated from each other. The resulting problem is that the detector as a whole is enlarged. Another problem is: Optical fibers made of resin are prone to attenuate smoke signals, while optical fibers made of glass are expensive because of a high price of the material.
Furthermore, the photoelectric smoke detector using optical fibers requires filling the portions of the explosion-proof enclosure to which the optical fibers are attached with resin in order to prevent penetration of explosive gas into the explosion-proof enclosure. Therefore as the filling resin deteriorates, the sealability of the explosion-proof enclosure is decreased, which may result in a high risk of explosion.